Find the one.
Make it a hundred thousand.
A dendritic cell arrives at a lymph node carrying a fragment of antigen. Inside, naive T cells are packed tight โ but only about one in a million can recognise that fragment.
And yet it gets found, within days. That one then becomes hundreds of thousands of killer T cells and leaves for the tumour.
These 20 lessons follow those days at the level of molecules โ the search problem, maturation and danger signals, signals 1, 2 and 3, the immunological synapse, autocrine IL-2, the Rb brake, clonal expansion. Python runs right inside this page.
How you'll learn
There are 10ยนโธ possible TCRs and only billions of T cells โ a hundred million times more kinds than cells. And yet the right one is found. We calculate why.
The TCR alone does not switch a T cell on. Without the second signal it switches off instead โ tolerance. We work out why it is built that way.
Doubling every ten hours, twenty times over, is a million. You count out why that looks slow at first and then suddenly is not.
Not a made-up example. We follow what actually happened to the one T cell that recognised nine letters โ SVDGQVYIP.